AMG 211 is a bispecific single-chain antibody construct of the bispecific T-cell engager (BiTE®) class that targets human carcinoembryonic antigen (CEA, CD66e) on (tumor) cells and cluster of differentiation 3 (CD3) positive T-cells. AMG 211 is a potentially new targeted drug in the treatment of relapsed/refractory gastrointestinal adenocarcinoma, since those are CEA expressing tumors. A well-known challenge in current drug development using targeted therapies is the high level of heterogeneity of target expression that is present in specific tumor types. Radio-labeling of AMG 211 with the positron emission tomography (PET) radionuclide Zirconium-89 (89Zr) enables non-invasive imaging and quantification of AMG 211 distribution in cancer patients. By performing a 89Zr-AMG211 PET scan prior to treatment with AMG 211, the uptake of the tracer in the primary and metastatic tumor lesions and normal organ distribution can be evaluated. By performing a 89Zr-AMG211 PET scan during AMG 211 continuous intravenous (cIV) treatment the investigators will be able to evaluate the impact of prolonged steady state exposure of AMG 211 on tumor and tissue uptake. The 89Zr-AMG211 PET imaging study may help to identify patients more likely to benefit from AMG 211 therapy.